Some health care systems and ride-sharing and ride-hailing services such as Uber and Lyft have formed partnerships to make it easier for these patients to get to appointments.

But the University of Pennsylvania study suggests this approach alone may not make a difference.

"Transportation is often a barrier to care for many patients, but solutions that don't address other barriers may not be enough to help patients get to doctor appointments," said study lead author Dr. Krisda Chaiyachati. He's a Veterans Affairs advanced fellow at Penn Medicine.

The study included nearly 800 Medicaid patients in Philadelphia, half of whom were offered free Lyft rides to two primary care practices. Medicaid is the publicly funded insurance program for the poor. Half the patients were younger than 46.

The missed appointment rates were nearly identical -- 36.5 percent for those who were offered the ride service and 36.7 percent for those who were not, the researchers reported.

"While it may be a negative finding, it's an important one because it can inform future efforts to help improve attendance rates, and highlights the complexity of social barriers when caring for poor patients," Chaiyachati said in a university news release.

A prior study found that around 3.6 million adults in the United States miss medical appointments each year because of transportation problems. Many of those patients are low-income, research suggests.

"One of the takeaways here is that we need to be thoughtful about how we design and test new programs that address social barriers to health care," said study senior author Dr. David Grande, an assistant professor of medicine at the University of Pennsylvania's Perelman School of Medicine.

"While we want hospitals and health systems to address patients' social challenges that impact health -- we need to rigorously evaluate new programs to make them successful," he added.

"In this case, addressing transportation alongside other barriers could make a difference or doing a better job identifying who needs the services," Grande said.